Better layout for xfwm4 tweaks compositing page

Submitted by Nikita Zlobin

Assigned to Olivier Fourdan @olivier

Link to original bug (#15863)

Description

Created attachment 8934 GtkBuilder UI form file

This is proposal for better layout for xfce-settings / wmextra / Compositing page, where is option to turn on compositing with shadows/transparency settings.

This proposal is from this thread (I needed some feedback before proposal): https://forum.xfce.org/viewtopic.php?id=13273 Where it is already liked.

Form in this proposal has dublicated vblank-mode option, so that you could try it in glade preview, by leaving only one line visible. Though I'm asked to make dropdown below main checkbox, I'm inspired by some elements from elementary-os style, so left both variants for your choice.

Button box variant uses just GtkBox, as I don't see any advantages from GtkButtonBox, besides predefined grouping styles and heh, slightly bolder buttons.

Attachment 8934, "GtkBuilder UI form file":
xfwm4-wmtweaks-compositor.ui